1. Field of the Invention
The present invention relates to the field of measuring and manufacturing optical surfaces. In particular the invention relates to an interferometer apparatus for measuring an optical surface and/or a method for qualifying the optical surface by using the apparatus and/or a method for manufacturing an optical substrate having the optical surface by using the interferometer apparatus.
2. Brief Description of Related Art
The substrate having the optical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, or systems used for imaging structures of a mask (“reticle”) onto a radiation sensitive substrate (“resist”) in a lithographic method. The success of such an optical system is substantially determined by the precision with which the optical surface can be machined or manufactured to have a target shape. In such manufacture it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface is then further machined at those portions where differences between the machined and target surfaces exceed e.g. a predefined threshold.
Interferometric apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477, which documents are incorporated herein by reference.
A conventional instrument and method for qualifying a convex optical surface will be illustrated with referenced FIG. 1 below. An interferometer apparatus 1 is of a Fizeau interferometer type having an optical axis 3 and a laser light source 5, such as a Helium-Neon-gas laser, emitting a laser beam 7. A microobjective 9 collimates laser beam 7 onto a pinhole of a spatial filter 11 such that a diverging beam 13 originates from the pinhole of spatial filter 11. A collimating lens 15 or a similar arrangement of plural lenses collimates diverging beam 13 to form a parallel beam 17. Wavefronts in parallel beam 17 are substantially flat wavefronts. A collimating lens 19 or similar collimating arrangement of plural lenses transforms parallel beam 17 into a converging beam 21 such that a crossover of converging beam 21 is formed in a region 23 on optical axis 3. Wavefronts in converging beam 21 are substantially spherical wavefronts.
A concave surface 25 of focussing lens 90 forms a reference surface or Fizeau surface of Fizeau interferometer 1. Concave surface 25 has a substantially spherical shape with crossover 23 coinciding with its center. Surface 25 is partially reflective, and spherical wavefronts of converging beam 21 are partially reflected from surface 25 such that they travel back in the beam path of interferometer 1 and are transformed to substantially flat wavefronts by focussing lens arrangement 19. A partially transmissive mirror 27 is arranged in the beam path of diverging beam 13 such that wavefronts reflected from reference surface 25 are imaged by a camera lens 29 onto a light sensitive substrate of a camera 31.
An optical substrate or a lens 33 to be manufactured has a convex spherical surface 35 to be qualified. Lens 33 is arranged in the beam path of converging measuring light beam 21 such that surface 35 faces interferometer optics 14 and such that a center of curvature of convex surface 35 substantially coincides with crossover 23. Spherical wavefronts of measuring light beam 21 are reflected from surface 35 and travel back through interferometer optics 14 and are imaged on camera 31. On camera 31 the wavefronts reflected back from reference surface 25 and the wavefronts reflected back from surface 35 under test are superimposed and form interference fringes detected by camera 31. From a measurement of such interference fringes deviations of surface 35 from its target spherical shape may be determined. Based on such determination surface 35 may be machined for better conforming to its target shape.
An opening ratio k may be defined for surface 35 having a diameter D, a radius of curvature R and an opening angle α as
                              k          =                                    1                              2                ⁢                sin                ⁢                                                                  ⁢                α                                      =                          R              D                                      ,                            (        1        )            wherein D=2 R sinα for α<90°.
If surface 35 is an aspherical surface, the definition of equation 1 may be maintained by defining the radius R with respect to a best approximating sphere of the aspherical surface 35.
It appears from FIG. 1, that for small values of the opening ratio k a diameter of lens 19 of interferometer optics 14 has to be substantially higher than diameter D of lens 33 under test. Manufacture of interferometer optics having a high diameter with the necessary precision is not only expensive but also demanding from its optical design.